1. Field of the Invention
The present invention relates to a semiconductor memory device and a control method of the same, and is applied to, e.g., a NAND flash memory.
2. Description of the Related Art
A NAND flash memory as an example of a semiconductor memory device includes a series circuit of a plurality of memory cells as a basic unit (NAND unit). The NAND unit has one end connected to a bit line via a select gate (SGD), and the other end connected to a source line via another select gate (SGS).
Data read and write operations are performed for each unit called a page connected to one word line. A set of pages sandwiched between the select gate (SGD) on the bit line side and the select gate (SGS) on the source line side forms a block.
Data is stored in the NAND flash memory by injecting electrons into or releasing electrons from a floating gate, thereby fluctuating the threshold value of a memory cell. The floating gate is insulated as it is surrounded by, e.g., an inter-gate insulating film as an insulating film. Therefore, even when the power supply is turned off after electrical injection or release, electrons are held in the floating gate in a nonvolatile manner.